-Price, nano clones can be bought for less than 2 pounds.
-I/O max output current (25mA preferred to drive leds and mosfets)
-All of the fancy stuff(ADC, PWM, SPI, I2C, Serial, etc.)
Look into an Atmega1284
I am curious which board(s) best meet the following criteria:-Compatibility with Arduino IDE and libraries-Fast loop time (even for big projects with displays and such)-Price, nano clones can be bought for less than 2 pounds.-I/O max output current (25mA preferred to drive leds and mosfets)-Size, Arduino nano, maple mini, and the pill boards have a compact form factor-All of the fancy stuff(ADC, PWM, SPI, I2C, Serial, etc.)-Power consumption(not important, just for completeness sake)
the vendor doesn't state whether it's the 160Mhz or 240Mhz version
You choose the speed when you upload the sketch. Same for esp8266 (80MHz or 160MHz).I would choose Maple Mini over black/blue pill boards. They come with bootloader installed and you don't have to worry about getting one with an incorrectly manufactured usb circuit that needs tiny resistors replacing.Also consider AdaFruit ItsyBitsy M0 and M4.
Don't get too caught up on "fastest", without having an exact spec for what you'll be doing with it.The M3/M4 boards are definitely more powerful than M0 boards, and for maximum compatibility I'd go with one of the Teensy 3.x boards. They've been out the longest, and PJRC goes to a lot of effort to ensure Arduino compatibility (as well as contributing to the Arduino code base.) 2nd choice would probably be the Adafruit "M4" boards - they are also well invested in the Arduino code and compatibility, but the boards are a bit "young" and still working out some bugs. (on the plus side - several different form factors!)Neither of these is likely to show up as a cheap clone, though. Expect to pay the full $20-$40 price...
The advantage of the samd21 boards, whoever they are made by, is their Zero/M0 compatibility, which means that support, libraries etc will be created & maintained by, or with the blessing of, Arduino Corp. None of them are as cheap as you would like, I suspect, because the Chinese board manufacturers have to pay ATMEL for the samd21 chips and, as yet, cannot make copies of them.The Teensy boards also benefit from high Arduino compatibility and support because of one man, a man with a high level of expertese and involement with the Arduino movement for a very long time. (But of course that could also be seen as their weakness: its a one-man band, I suspect. What if he got hit by a buss tomorrow?) Have you checked out the Teensy LC? Only slightly more expensive than the cheap Chinese samd21 boards.Also check out the Tau. This does use a slightly different chip to the Zero and compatible: the slightly smaller and cheaper samd21e. Is not quite 100% compatible I believe, so that's a small risk.The best "bang-for-buck" in terms of processing power is probably the esp chips/boards.